Thermostatic shower with poor flow

This may help you diagnose a problem with a shower that doesn't seem to have enough pressure. You can skip to the end and try the suggestion to see if it works, or read through the explanation if you have time. Note that this applies to UK plumbing.

Symptom - Shower flow is feeble, particularly on hot, but the problem may be intermittent or appears gradually.

When we moved house a few years ago, there was a thermostatic shower in the en-suite that seemed to work fine to start with, but gradually lost pressure, and after a few years the hot water reduced to just a trickle. However, we found it would occassionally fix itself, so lived with it for a while until we replaced the bathroom, and I decided to fit a new shower. Unfortunately, after fitting the new thermostatic shower, I found it was even worse than before

Pipework to a gravity fed shower.  Illustration only.

The hot supply was from the hot-water cylinder which was on the same floor, and the cold was from the cold-water tank which was in the roof. The hot-water cylinder is pressurised by the supply from the cold-water tank, so normally the hot and cold water should arrive at the shower at about the same pressure. It doesn't really matter where the hot-water cylinder is, the key thing is the vertical height between the cold-water tank and the shower. If the tank is up in the roof there shouldn't be any problem, it normally just needs about 1m between the bottom of the tank and the shower. This diagram shows the basic layout, but please don't use this to do any actual plumbing, there are a lot of parts not shown.

Air collects in long horizontal pipe runs

Basically everything was fine, and there should have been plenty pressure to allow a good flow at the shower. The problem I had was that whoever had put in the original plumbing had run the hot water pipe up into the roof, then flat along the rafters for about 10m, then back down to the shower. Since this long run of pipework is at the highest point, and is flat, it collects air, and as can be seen from the diagram this will cause the flow to collapse.

Solution for poor shower flow